Geology Reference
In-Depth Information
Site 3-97
Site 2-97
Columnar ice
Site 1-97
Frazil ice
3 km
Figure 4.33 ERS‐2 SAR image of Allen Bay (central Arctic)
acquired in May 1997 showing three areas of FY ice with dif-
ferent backscatter signatures [ Johnston, 1998].
brine. The diversity of the crystallographic composi-
tion and texture as well as surface roughness of FY ice
is reflected in its signatures from all bands of remote
sensing observations. Figure 4.33 shows different back-
scatter signature of FY ice in Allen Bay (a small bay
near Cornwallis Island, Canadian central Arctic)
detected by the synthetic aperture radar (SAR) on
ERS‐2. The land‐fast FY ice in the scene is divided into
three distinct regions (marked as sites 1, 2, and 3) with
different crystallographic structure, surface roughness,
and consequently radar backscatter (shown as gray tone
in the image). Ice in site 1 has relatively smooth surface
while ice in site 2 has rough surface with a few MY ice
floes entrapped. Site 3 is located at the estuary of a
small river, so it features ice with less salinity (hence less
scattering for the same surface roughness and therefore
darker signature). Johnston [1998] presented the ice
microstructure and some physical properties from each
site (Figure  4.34). The ice in site 1 has three layers of
frazil ice within the top 8 cm. As the ice started to con-
geal in the top two layers, it was interrupted by frazil
deposit at the bottom. Finally, it congealed into narrow
columns of ice. The salinity at the surface is about 16‰,
yet it declines to about 4‰ at depth of 6 cm. The FY ice
in site 2 features elongated frazil structure, which
started to congeal at 10 cm depth. The salinity fluctu-
ated between 7‰ and 10‰. Ice in site 3 froze under
steady conditions. Columnar ice started to form imme-
diately after initial freezing with no frazil ice observed
(Figure  4.47). The salinity profile is near constant at
8‰ for the examined depth. This example shows the
possible spatial variability of FY ice microstructure
within a small area.
Figure 4.32 Ice core segment between 0.15 and 0.23 m
depth, from Lancaster Sound, Canadian Arctic, in 1992 after
cutting it vertically into two halves. It shows the interruption
of columnar ice growth by deposition of frazil ice at 23 cm
depth (photographed by M. Shokr).
ice formation and growth. Since ice does not grow very
thick in these areas (average ice thickness in the middle of
winter in the Gulf of St. Lawrence in the east coast of
Canada is around 0.50 m), ice congelation is less likely to
occur in those areas. This contrasts the crystallographic
texture of sea ice in the polar regions where ice most often
congeals into columnar structure at depths not far from
the surface. Nevertheless, even in the Arctic, columnar
(congealed) growth of ice may be interrupted by a frazil
layer if the latter is carried away by ocean current and
“stick” to the bottom of the existing ice sheet. Ice may
continue its growth and re‐congeal at lower depths. This
situation is illustrated in Figure 4.32, which shows a pho-
tograph of an ice core segment at depths between 0.15
and 0.23 m obtained from Lancaster Sound, Canadian
central Arctic, in May 1992. A layer of frazil ice appears
to underlie a layer of columnar ice (both layers are con-
firmed in thin section photographs).
Due to the diversity of ice formation and growth con-
ditions, the crystalline structure of FY ice cannot be cat-
egorically characterized. All crystallographic ice classes
(section  4.3.1.) are found in FY ice. Perhaps the only
feature that characterizes the microstructure of FY ice
is the presence of a large number of brine inclusions in
addition to small air bubbles found mostly inside the
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